The present invention relates to a semiconductor integrated circuit and a liquid crystal display device, and particularly to an improvement to be effectively applied to a picture signal line driving circuit (drain driver) of a liquid crystal display device which is capable of displaying many gradations.
An active-matrix liquid crystal display device having an active element (e.g. thin film transistor) for each pixel and which operates by switching the active element is widely used as a display device of a notebook-type personal computer and the like.
Because the active-matrix liquid crystal display device applies a picture signal voltage (a gradation voltage corresponding to display data; hereinafter referred to as a gradation voltage) to a pixel electrode through an active element, no crosstalk is generated between pixels, and so it is unnecessary to use a specific driving method for preventing crosstalk, such as is necessary in a simple matrix liquid crystal display device, thereby making it possible to display many gradations.
As an active-matrix liquid crystal display device, the following devices are known: a TFT (thin film transistor)-type liquid crystal display panel (TFT-LCD) and a TFT-type liquid crystal display module provided with a drain driver set to the upper side of a liquid crystal display panel, a gate driver set to the lateral side of the liquid crystal display panel, and an interface section.
In general, when the same voltage level (DC voltage) is applied to a liquid crystal layer for a long time, the inclination of the liquid crystal layer becomes fixed, causing an afterimage phenomenon, which operates to shorten the service life of the liquid crystal layer.
To prevent the afterimage phenomenon, in the case of a TFT-type liquid crystal module, the voltage to be applied to a liquid crystal layer is changed to a periodically changing voltage like an AC voltage, that is, the voltage to be applied to a pixel electrode is changed between a positive voltage and a negative voltage periodically in accordance with the voltage to be applied to a common electrode.
To apply an AC type voltage to the liquid crystal layer, the following two methods are known: the common symmetry method and the common inversion method. The common inversion method is a method wherein the voltage to be applied to a common electrode and the voltage to be applied to a pixel electrode are alternately changed between a positive voltage and a negative voltage. On the other hand, the common symmetry method is a method wherein the voltage to be applied to a common electrode is kept constant and the voltage to be applied to a pixel electrode is alternately changed between a positive voltage and a negative voltage relative to the voltage to be applied to the common electrode.
According to the common symmetry method, it is possible to use the dot inversion method or the V-line inversion method, which provides a small power consumption and a superior display quality.
The above-described technology is disclosed in U.S. patent application Ser. No. 08/826,973 filed on Apr. 9, 1997, now U.S. Pat. No. 5,995,073.
In the case of the dot inversion method, as shown in FIG. 30, the gradation voltage (VDH) to be applied to the odd-numbered drain signal line (D) and the gradation voltage (VDL) to be applied to the even-numbered drain signal line (D) have opposite polarities relative to the driving voltage (VCOM) to be applied to a common electrode. That is, when the gradation voltage (VDH) to be applied to the odd-numbered drain signal line (D) has a positive polarity (or negative polarity), the gradation voltage (VDL) to be applied to the even-numbered drain signal line (D) has a negative polarity (or positive polarity). Moreover, the polarity is inverted for each line and the polarity for each line is inverted for each frame.
FIG. 30 is a signal diagram showing the relation between the gradation voltage to be applied to a drain signal line (D), that is, the voltage to be applied to a pixel electrode and the driving voltage (VCOM) to be applied to a common electrode. The gradation voltage to be applied to a drain signal line (D) shown in FIG. 30 is for displaying black on the display screen of a liquid crystal display panel.
Thus, the dot inversion method has a disadvantage in that the chip size of a drain driver increases because a circuit for generating positive- and negative-polarity gradation voltages is necessary for each drain signal line (D).
To avoid the above-described disadvantage, in the case of the TFT-type liquid crystal display module disclosed in U.S. patent application Ser. No. 08/826,973 filed on Apr. 9, 1997, now U.S. Pat. No. 5,995,073, the chip size of a drain driver is decreased by using the fact that the polarity of the gradation voltage (VDH) to be outputted to the odd-numbered drain signal line (D) is always opposite to that of the gradation voltage (VDL) to be outputted to the even-numbered drain signal line (D) in the case of the dot inversion method, thereby making it possible to share a circuit for generating positive- and negative-polarity gradation voltages for two drain signal lines (D) by switching the circuit using a switching section, resulting in reduction of the chip size.
However, the TFT-type liquid crystal display module disclosed in U.S. patent application Ser. No. 08/826,973 filed on Apr. 9, 1997, now U.S. Pat. No. 5,995,073, has a problem in that a transistor having a higher withstand voltage between source and drain is necessary as the switching transistor of the switching section, thereby increasing the chip size of the drain driver, when it is necessary to increase the gradation voltages (VDH and VDL) to be applied to a drain signal line (D), compared to those of a conventional TFT-type liquid crystal display module, due to change of the materials of the liquid crystal layer.
In the case of a liquid crystal display, such as a TFT-type liquid crystal display module, the display screen has been increased in size, and the tendency is for the display screen size to even further increase. Moreover, to eliminate unnecessary space and improve the fine view provided by a display, it has been proposed to decrease the region outside of the display region of a liquid crystal display, that is, minimize the frame portion (frame minimization).
However, when the chip size of a semiconductor integrated circuit (IC chip) constituting the drain driver increases by using a transistor having a higher withstand voltage between source and drain as the switching transistor of the switching section, a problem occurs in that it is impossible to deal with the requirements for frame minimization.
The present invention has been made to solve the above problems, and its object is to make it possible to use a transistor with a low withstand voltage for a semiconductor integrated circuit operating as the switching element of a switching circuit in which a voltage higher than the normal withstand voltage between the source and drain of the transistor with low withstand voltage is applied between input and output terminals.
It is another object of the present invention to make it possible to use a transistor with a low withstand voltage for a liquid crystal display operating as the switching element of a switching section in which a voltage higher than the normal withstand voltage between the source and the drain of the transistor with low withstand voltage and output positive- and negative-polarity picture signal voltages to a pair of picture signal lines without increasing the chip size of the picture signal line driving means.
The above objects and novel features of the present invention will become more apparent from the following description and the accompanying drawings.